Nanometre fcc clusters versus bulk bcc alloy: The structure of Cu-Pd catalysts

We present a simple method to improve the accuracy of the calculated heat of mixing for the Cu-Pd alloy within the formalism of the molecular dynamics/ Monte Carlo-corrected effective medium (MD/MC-CEM) theory by adding a fitted Morse potential to the pair interaction between Cu and Pd atoms. This leads to a much better agreement between the theoretical and experimental values of heats of mixing for five different compositions of the Cu-Pd alloy in the bulk phases. Using this newly fitted model, we have performed simulations on CuPd clusters consisting of 50–10000 atoms with fee and bcc structures. Our calculations show that in the range of cluster sizes of several thousand atoms, the fee structure is energetically favoured over the bcc structure. We estimate an approximate size for the fee to bcc (CsCl, known bulk structure for CuPd) transition in these clusters to be around 10000 atoms. Additionally, we have also performed calculations of the X-ray diffraction patterns of a variety of cluster geometries and sizes. The calculated X-ray diffraction pattern of a slightly distorted fcc cluster exhibits the main features observed in the available experimental diffraction patterns of colloidal bimetallic catalysts of CuPd. The calculated diffraction patterns of bcc clusters are quite different from the experimental data.